Cannot Select Linked When Sync Selection Is Enabled
That's nothing about UV in this case. That's because you were trying to tweak pose in Rest position. Just remember to simply switch to Pose Position to continue tweaking. :)

Location:
Properties Editor > Object Data Tab > Skeleton > Pose position



BTW, there are indeed two such messages, located in:


source/blender/editors/transform/transform_conversions.c
source/blender/editors/uvedit/uvedit_ops.c


True, it's a strange message for bone transform system, which seems hard to make sense, probably needs to be improved.



Edit:

In the latest build, this puzzling message has been fixed as "Cannot change Pose when 'Rest Position' is enabled". So it is not an issue any more.